Measuring system with capacttor having characteristics of an infinite capacity



Jan. 18, 1949. WILLIAMS, JR 2,459,730

MEASURING SYSTEM WITH CAPACITOR HAVING CHARACTERISTICS OF AN INFINITECAPACITY Filed June 30, 1944 I nvmvrozc 194 MErJMu/flmJe.

(NEW

Patented Jan. 18, 1949 r iorr cs MEASURING SYSTEM WITH CAPACITOR HAVINGCHARACTERISTICS, OF AN IN- FINITE CAPACITY Albert J. Williams, Jr.,Philadelphia,

to Leeds and'NorthruD Company,

Pa.-, assignor Philadelphia,

Pa., a corporation of Pennsylvania Application June so, 1944, Serial No.542,899

This invention relates to measuring systems and-has for an object theprovision of relatively simple, though reliable, measuring systems ofthe electronic type in which exceedingly high accuracy may be attainedin the measurement of direct currents or direct current potentials oflow magnitude.

More particularly, electrical energy, in the form of low frequencyalternating current or a direct current electromotive force, produced inresponse to a variable condition (such as voltage, current, temperature,or the'like) is repeatedly interrupted in efiect to convert theappliedenergy into alternating current. The alternating current electromotiveforce, so derived, is applied to the input of an amplifier having anoutput circuit which includes means for rectifying the amplifiedalternating current. The rectified current is then utilized to producein the in tude may be of a very low order. For example, it

may be only a few microvolts.

Direct current amplifiers have heretofore been proposed for measurementof unidirectional electromotive forces, but they are not satisfactoryfor the amplification and measurement of electromotive forces of themagnitude produced by thermocouples and other similar low potentialcondition-responsive devices. Deflection in struments of greatsensitivity have also been used for measurement of potentials of smallmagnitude but instruments of this character are not suitable wherevibration and mechanical shocks are unavoidable. V

In accordance with the present invention, measuring systems are providedwhich are not 7 Claims. (01. .171-95) mechanical systems, the residualunbalance present may be due to the lost motion between, or only to theflexure of, the parts. But such a residual unbalance is enough toreflect in a lowered accuracy of the system. In electrical systems ofthe type heretofore proposed, a variable potential applied to the inputcircuit of a direct current amplifier produced an output current which,through a resistor, produced a potential difference which opposed theinput voltage. An ammeter in the output circuit was then calibrated interms of the input voltage.

For applications where such systems were othersubject to shock orvibration and which amplify and accurately measure direct currentpotentials of exceedingly small magnitude.

In accordance with a further aspect of the invention, there is reducedto a negligible degree the residual unbalance which has heretofore beenpresent in measuring systems of the null type. Though such residualunbalance may be small, it nevertheless limits the ultimate accuracy ofa measuring system. For example, in

wise satisfactory, there was, nevertheless, present a residual unbalancedue to the fact there must always exist a slight-potential differencebetween the applied potential and that fed back in order to insure thatthe amplifier would maintain the new value of output current. In otherwords, there must always be a proper and sumcient potential diflferencebetween the grid and cathode to maintain the needed value of outputcurrent. r

In order to apply the signal voltage to the input circuit of athermionic amplifier, integrating capacitors have been used. The voltageacross such a capacitor is a function of the integral with respect totime of the current flowing into the capacitor. When this current stopsflowing, the capacitor tends to lose its charge due to discharge throughits own charging circuit. This also gives rise to residual unbalance.Such losses must be supplied by the input or charging circuit.

In carrying out the present invention in one form thereof, the inputsignals whose magnitude is to be measured are modulated to produce analternating current signal of readily amplifiable frequency. The A. C.signal after desired amplifiction is then demodulated to produce anoutput signal of character identical with that of the applied signalsbut of amplified magnitude. The output signal is utilized to produce inthe input circuit a voltage whose instantaneous polarity opposes that ofthe applied signals. Hence, the magnitude of the amplified anddemodulated signal becomes a measure of the magnitude of the appliedsignal. Hence, a. rugged, vibration and shockproof, measuring instrumentmay be, and

is, used for measuring the magnitude of the demodulated output. Thisinstrument is preferably calibrated directly in terms of the magnitudeof the condition under measurement.

In accordance with a still further aspect of the present invention, agreater degree of amplification is attained with a stability of theable.

. w 3 amplifyingcircuits which has not heretofore been achieved.

It'is well understood by those skilled in the art that a cascade ofamplifying devices will produce any desired amount of amplification.However, as the gain is increased, the circuit and tube capacities, theinductive and. extraneous fields,

all tend tomake the amplifier unstable. From such causes, parasiticoscillations are unavoid- Such parasitic oscillations overload the tubesand thereby produce distortion, and hence, inaccuracy-in the measurementof the applied signals.

In carrying out this further aspect of the invention, an alternatingcurrent amplifier is utilized to produce an amplified output 01 amagnitude within the capabilities of an essentially stable amplifier.and applied to an integrating capacitor connected across the inputcircuit of a direct current amplifier. This second amplifier serves tofurther amplify the applied signals to within the capabilities of astable direct current amplifier. Hence,

, without distortion, the original signal may be The output is thenrectified plate of the capacitor is to the same degree made morenegative. This makes possible the maintenance of substantially groundpotential on the grid or input side of the capacitor. Hence, there isbut little, a negligible, tendency of the capacitor to lose its chargethrough the input or supply circuit. Moreover, upon a rise in voltageacross tion, and for further objects and advantages thereof, referenceis to be had to the following detailed description, taken in conjunctionwith the accompanying drawings, in which:

Fig. 1 is a wiring diagram which diagrammatically illustrates one form,of the invention; and

Fig. 2 is a wiring diagram diagrammatically illustrating another form ofthe invention.

Referring to the drawings, the invention in one form has beenillustrated as applied to the measurement of temperature to which athermocouple Ill--may be subjected, although it is to be understood theinvention is applicable to the measurement of any direct-current orpotential which varies with, or as a function of, the magnitude of acondition, such, for example, as pressure, rate of flow, voltage,current, power, or the like.

As well understood by the art,.the thermocouple III, in response to thedifference in temperature between its hot junction and its coldjunction, produces a potential difference. This potential difference orelectromotive force is applied across a circuit including conductors IIand l2. l3, l4 and I5 and capacitors I6, I! and [8, which together forma filtering circuit for by- One side of the circuit includes resistorspassing to ground G alternating current which may be induced in, orpicked up by, the foregoing circuit. It will be observed the conductorl2 leads to the cathode of the first stage or tube 20 01a thermionicamplifier 2|, this connection being completed through a conventionalresistor-capacitor cathode-biasing means 22. The other side of thecircuit extends from the resistor It to a stationary contact 23 of acircuit maker and breaker, or vibrator 24, operable by a coil 25 toconnect the grid of the tube or valve 20 first to the contact 23 andthen to another stationary contact 26. The operating coil 25 isenergized from-a suitable source of alternating current 21. The inputcircuit to the valve 23 includes a coupling capacitor 23 and there isalso provided a grid leak and shunting capacitor 33.

The stationary contact 26, when engaged by the vibrator 24, connects theinput circuit ofvalve 20 across another circuit whichincludes a resis-.tor 32, by which there is produced a voltage or electromotive force forbalancing the voltage or potential difierence produced by thethermocouple Hi. This branch circuit also includes .a filter comprisingresistors 33, 34 and 35 and capacitors 36, 31 and 38. This filter alsoserves to bypass to ground G, by way of conductors 40, 4| and 42, anyalternating current which may appear in this circuit. I

It will be observed the vibrator 24 performs several functions. First,it serves to interru t the direct current or unidirectional potentialawlied by the thermocouple ID to the input circuit of the valve 20. Bycontinuously making and breaking this circuit, the direct current isconverted to a pulsating current which, so far as the valve 23 isconcerned, produces operation like that which valve 20 responds to thedifference between the potential of the thermocouple l0 and that acrossthe resistor 32. The valve 20,-serving as the first stage of theamplifier 2|, amplifies or magnifies the resulting potential differenceand applies it through acoupling capacitor 44 to the amplifier 2|. Theamplifier 2| preferably includes a plurality of stages of amplificationfor greatly increasing the magnitude of the otential difference orsignals applied to the foregoing input cir-' cuit. These amplifiedsignals or potential difierences are-then applied to the primary windingof an output transformer 45, the secondary winding of which has itsrespective ends connected to the stationary contacts 46 and 41 of asecond vibrator 48. This vibrator is operated by means of a coil 49 fromthe same source 21 of alternating current. The vibrators 24 and 48operate in synchronism. The secondary winding of the transformer 45 is.shunted by a resistor 50. From a center tap oithe resistor 53 thereextends a conductor 5| which leads to one side of the resistor 32.

The vibrator 43 serves to complete the foregoing circuit to the resistor32 by way of a conductor 52, resistor 53, a current-responsive de- 4| tothe other side of resistor 32. The vibratoi' 48 serves or functions asa, full wave rectifier. Since it operates in synchronlsm with thevibrator 24 and inasmuch as the unidirectional or direct currentpotentials applied to the input circuit are converted into alternatingcurrent,

it will be understood that the movement of the vibrator 48 to complete acircuit through the contacts 46 and 41 first to one side of thesecondary winding and then to the other side of the secondary winding ofthe transformer 45, produces in the circuit, including resistor 32, aunidirectional or direct, current. If the resistor 50, is omitted, it isto be understood the conductor 5| would then be connected to themid-point of the secondary winding of transformer 45.

In order to stabilize the operation of the system as a whole, and topermit high gain in the amplification with avoidance of oscillationswithin the system, a. capacitor 51 is connected across the vibrator 48and the midpoint of resistor 50. The

capacitor 5'! is large in capacity, of the order of IQOO microfarads, ascompared with the shunting or parallel capacities of the'other elementsof the circuit, whether these capacities be interelectrode capacities orthose included in the circuit for other reasons, such as the filtercapacitors. More specifically, the capacitor 51 and the time constant ofthe associated circuit, including the resistor 53, of the order of 1000ohms, is large compared with the time constant or size of the capacitiesof each filter, including the resistors 33-35 and the capacitors 35-48.

With a system constructed and operated in accordance with the foregoingdescription, the unidirectional potential difference produced by thethermocouple It] produces inthe output circuit, including the meter 54,a direct current of relatively large magnitude. This current in flowingthrough the resistor 32 produces a potential difference which, asapplied to the input circuit of the tube 20, opposes the appliedpotential difference applied by the thermocouple ID. If the temperatureto which the thermocouple I0 is subjected increases, there is acorresponding increase in the potential difference produced thereby.'Ihis is reflected by a substantial increase in the output currentthrough the circuit including the meter 54.

a current, flowing through the resistor 32, increases the balancingpotential produced thereby and this increase is of proper magnitude tobalance the increased potential due to the aforesaid temperature rise.

It will now be apparent that the meter 54 may be calibrated interms ofthe magnitude of the condition under measurement. For example, it may becalibrated in temperature. Its scale may be either in degreescentigrade' or degrees Fahrenheit.

If the temperature of the thermocouple l0 decreases, the electromotiveforce produced thereby also decreases. The potential difference acrossthe resistor 32 is then momentarily higher. Hence, the relativepolarities of the applied signals are reversed and, in terms ofalternating current, there is a phase shift of 180. These same changesare reflected in the output circuit.

However, this increased thermocouple l0, the corrective actiondisappears. Because of the action produced by a polarity change or phaseshift, this form of the invention may be applied to input signals 0unidirectional or alternating character.

In contrast with direct current electronic voltmeters and with directcurrent amplifying systems of the prior art, .the permissibleamplification and'stability of the present system are materially greaterthan previously achieved. The

system is not subject to vibration or magnetic disturbances except to arelatively small degree. The conversion of the direct current potentialto alternating current greatly facilitates the design of the circuits.They are more efiicient, more stable, and the overall results arematerially and substantively better than heretofore achieved. Morespecifically, the milliammeter 54, of rugged construction, accuratelymeasures to within five microvolts, that is, to within one percent offull scale, for input voltages of the order of from zero to 500microvolts. It is to be further understood that the thermocouple Hf isillustrative of but one form of condition-responsive device. Devices ofother types may be used and the input signals to the amplifier may beeither direct or unidirectional in character'or they may be alternatingwith a frequency below thatwhich may be readily amplified. Forfrequencies which may not be readily amplified, the output'from theamplifier is demodulated so that the instantaneous voltage drop acrossresistor 32 is always the same as that of the applied signal or theelectromotive force whose magnitude is to be measured, as regardsmagnitude and polarity.

In accordance with the invention, the advantages of a null type systemhave been retained with elimination of the need for slidewires, standardcells, batteries, battery rheostats,

is applied to the input circuit of an integrator stage represented bythe valve 6|. This input circuit includes a resistor 62. If thisresistor 62 is omitted, the input circuit may be connected directly to acurrent source, in which case the capacitor 60 would also be omitted.The other circuit connections would remain unchanged.

Whether the input signals or voltages be low frequency alternatingcurrent or direct current, the conversion thereof to alternating currentof readily amplifiable frequency is accompanied by the production of awide range of harmonics. The accompanying harmonics may be eliminated inconventional manner, as by filters or tuned circuits. Their presence,however, limits the overall gain of the amplifier if stable operationwithout parasitic oscillations is to be maintained. Such stableoperation is most desirable in measuring systems and the gain of tube orvalve 20 plus that of the amplifier 2| is not-high enough to produceunstable operation. The additional gain which is desired is attained, bythe direct current amplifier 6|.

This amplifier 6| includes an integrating capacitor 04. It is connectedfrom the grid of valve- GI and from one side of the input circuitthrough the current-responsive meter 54, a resistor 65, and by theresistor 32 to the other side of the input circuit, the ground G, and tothe cathode of tube 6|. The capacitor 64 is fairly large. It may be ofthe order of four microfarads. It is preferably a high grade, minimumloss, capacitor. For example, it may be a high grade mica type ofcapacitor. In this connection, it is to be understood capacitors may bemade nearly perfect. The potential rise across the capacitor 64 isdirectly proportional to the time integral of the current flowing intoit. This potential difference or the voltage across the capacitor 84controls the bias or potential of the grid of the electric valve 6|. Asthis voltage rises, the grid becomes less negative, and more currentflows in the output circuit. As shown, I a suitable biasing means forthe grid, such as the batteryC, is provided to insure operation of thevalve 6| on the desired, or straight line, por'tion-ofdts characteristiccurve.

.. .The output circuitincludes asuitable. source of anode potential as.indicated'by the battery 61. The output current flows from the battery61, through the valve 6|, the resistors 32 and 65, the meter 54, andbyconductorfltto the'other side of the battery. The resistor 32functions as in Fig. 1 to introduce into the input circuit a balancingpotential. However, the resistors 32 and 65 perform a further andimportant function. The IR drop, or the polarity of the potentialdifference across the resistors 32 and 65, is in a direction such as tomake more negative the oathode side of the capacitor 64. The grid sideof the capacitor 64 operates at substantially ground potential but thecathode side thereof is negative to a degree dependent upon themagnitude of the current in the output circuit., This has the electricaleffect of preventing a reduction in the current flowing into thecapacitor, notwithstanding there is a voltage rise across the capacitor.Since the potential on the grid side of the capacitor rises but a smallamount, and is always near ground potential, there is little tendencyfor discharge of the capacitor through the supply circuit including theresistor 62. This resistor 62 is preferably large, of the order of twomegohms, and insures a minimum loss of potential by the capacitor 64.More particularly, as soon as the voltage across the capacitor 64 risesa small amount, the current through resistors 32 and 65 rises toincrease negatively the bias on the cathode side of the capacitor. Thisincrease of the negative bias on the cathode side of the capacitor takesplace rapidly and its magnitude changes accord- Specific values ofcircuit impedances will depend upon particular applications. Inaccordance with the system of Fig. 2, the capacitor 84 may have acapacity of four microfarads; the resistors 32, 65 and 62 may haveresistances respectively of 5 ohms, 100,000 ohms and 2 megohms.

While preferred embodiments of the invention have been described, itwill be'understood that further modifications may be made withoutdeparting from the spirit and scope of the invention as set forth in theappended claims.

What is claimed is:

1. In a-measuring system the combination of an electric valve having ananode, acathode and a control grid, an input circuit connected to saidgrid and to. said cathode, an integrating capacitor, means including aresistor between said capacitor and said cathode and between saidcapacitor and the cathode side of said input circuit for connecting saidcapacitor to said grid and to said cathode independently of the cathodeside of said inputcircuit, means connected to said input circuitfor'charging said capacitor in response to variations of a condition,the change in voltage across said capacitor determining the conductivityof said valve, an output circuit for said valve including said resistor,said resistor in response to current in said output circuit producing achange in potential on the cathode side of said capacitor opposite to achange in potential on the grid side thereof to compensate for a voltagerise across said capacitor so that the current flowing into saidcapacitor is not reduced because of said voltage rise.

2. The combinationwithan electric valve having an input circuit to whichsignals may be applied and an output circuit for flow of current underthe control of said input signals, 'of an integrating capacitorconnected across said input circuit for control of said output currentin response to an integration with respect to time of said inputsignals, and a resistor separate. from ing to the amplification factorof the tube or valve 6!. These features combine to make the residualunbalance, which has heretofore been present in order to maintain a newvalue of output current,'a minimum.

Further in accordance with the invention, it will be observed a zeroadjustment is provided by a resistor 10 and a battery 1|. This resistoris adjusted so that when there is zero input, the meter 54 will readzero. This provision is independent of the features already described.After adjustment of the zero position of the meter 54, the outputcurrent flowing through the meter 54 will be precisely related to thepotential or to the current being measured. The resistor 32 is a highprecision resistor, preferably of Manganin, so that its resistance willnot change with temperature or with changing current flow.

said input circuit and included in series circuit relation with saidcapacitor on the cathode side thereof and traversed by said outputcurrent in direction to impart to said capacitor the charac- I rent,said amplifier having an output circuit,

means included in said output circuit for rectifying said alternatingcurrent, a direct current amplifier having input and output circuits,means for minimizing residual unbalance in the system comprising anintegrating capacitor connected to said input circuit, means forapplying said rectified alternating current to said capacitor thereby tovary the charge on said capacitor, means included in both of saidlast-named input and output circuits and responsive to the outputcurrent for varying the potential on one side of said capacitor as itscharge is increased or decreased to impart to it the characteristic ofan infinite capacity, means in said first-named input circuit forproducing in response to said last-named output current an electromotiveforce of polarity opposite to said applied electromotive force, andmeans for measuring the magnitudeof said lastnamed output current toobtain an indication of the magnitude of said applied direct currentelecj tromotive force. 1 4. In a measuring system the combination anelectric valve having an anode, a cathodeand a control grid, an inputcircuit connected between said grid and said cathode, an integratingcapacitor having one side connected to the gridside of said inputcircuit, means including a resistor between said capacitor and saidcathode and'between said capacitor and the cathode side of said inputcircuit for connecting the other side of said capacitor to the cathodeside of said input circuit, means for charging said capacitor inresponse to variations of a condition, the resultant change in voltageacross said capacitor with reference to said cathode determining theconductivity of said valve, an output circuit for said valve includingsaid resistor, said resistor, in response to current in said outputcircuit producing a change in potential on the cathode side of saidcapacitor opposite to a change in potential on the grid side thereof sothat upon a voltage change across said cathode upon application of asignal to said i nput circuit, a resistor, means connecting saidresistor between said other side of said capacitor and said cathode sideof said input circuit, and means inciuding said resistor operable inaccordance with the output of said valve for producing with ref-' erenceto the cathode a change in potential on the cathode side of saidcapacitor opposite to any change in potential on the grid side thereofso that flow of signal current to and from said capacitor is independentof the magnitude of the I potential diflerence across said capacitor.

7. The combination with an electric valve having a source of anodesupply, at least an anode, a cathode and a control electrode, of anoutput circuit, a-signal input circuit connected between said controlelectrode andsaid cathode, a separate integrating circuit comprising acapacitor capacitor the current flowing into it is not changed.

5. The combination with an electric valve having an anode, a cathode anda control'grid, an output circuit and a signal input circuit betweensaid grid and cathode, of an integrating capacitor for said inputcircuit having one side thereof connected to the grid side of said inputcircuit, signal means for varying the charge of said capacitor, and aresistor between said other side of said capacitor and said cathode sideof said input circuit operable in accordance with the current output ofsaid valve for varying the potential on the cathode side of saidcapacitor as its charge I is increasedor decreased by signal input toimpart to it the characteristic of an infinite capacity.

6. In a measuring system, the combination of an electric valve having ananode, a cathode and a control grid, a signal input circuit connectedbetween said control grid and said cathode, an integrating capacitor oneside of which is connected to the grid side of said input circuit forchanging the potential of said grid relative to said 6 having one sidethereof connected to the grid side of said input circuit, a resistor inseries with said capacitor having the side remote from said capacitorconnected to the cathode side of said input circuit, said resistor as awhole being electrically between the cathode side of said input circuitand said capacitor, and conductors connecting said resistor for flowofoutput current therethrough in a direction to make the capacitor sidethereof negative with respect to said cathode.

ALBERT J. WILLIAMS, JR.

REFERENCES CITED The following references are of record in the file ofthis patent:

